In recent years, as a thin-film device to form an integrated circuit on a glass substrate, poly-Si TFTs (thin film transistors) have been vigorously developed. The poly-Si film is generally formed in an excimer laser crystallization technique. In this technique, an amorphous silicon (a-Si) film is once formed, and an excimer laser beam is irradiated onto the a-Si film to melt and re-crystallize the a-Si film, to form the poly-Si film. In the excimer laser crystallization technique, the molten state of the a-Si film depends on the film thickness thereof, optical constant of the film, wavelength of the excimer laser beam, energy density, pulse width, beam profile, and the like. In general, the laser irradiation process management deals with thickness of the a-Si film and energy density of the laser irradiation apparatus as targets to be managed. This is because the molten state which should essentially be managed is difficult to evaluate and manage.
In the molten state of the a-Si film, the depth of melt increases depending on the energy density of the laser beam. Significant phases in changes of the molten state appear at two points. One is the phase in which the film surface starts melting. The other is the phase in which the whole film completely melts once the melt depth reaches the full thickness of the film. The former and latter molten states bring about crystallization and micro-crystallization, respectively. Film temperature rises due to laser beam irradiation, and a part of the film melts. Then, the molten area or part is crystallized by later cooling. If the film does not completely melt, the nucleation sites during the crystallization in a solid-liquid interface. If, on the other hand the depth of melt equals the film thickness, the nucleation sites form in a-Si/substrate interface. In any case, crystallization is associated by heterogeneous nucleation. At this time, the grain diameter increases depending on the increase in the energy density. TFT characteristics and particularly the mobility thereof depend on the grain diameter. Hence, the grain diameter has been required to become as large as possible.
On the other hand, however, when a film completely melts, the mechanism of crystallization from a liquid phase, which has reached a thermal equilibrium state, changes into a homogeneous nucleation in which the nucleation occurs throughout the film. The grain diameter of crystal grains formed at this time is as small as 20 nm. A phenomenon of rapid reduction in the grain diameter which appears when energy density is excessively increased is called micro-crystallization. The energy density at which the micro-crystallization occurs is referred to as micro-crystallization threshold value. In a physical meaning, the micro-crystallization threshold value is a parameter which standardizes the change in film thickness, energy density, and the like together, and can evaluate the changes in the molten state. Besides, the micro-crystallization threshold value is a highly important value from the viewpoint of practical use that irradiation with higher energy density than the threshold value adversely reduces the grain diameter and degrades the TFT characteristics. It is to be noted that, as the film thickness decreases, the cooling period is shortened. Accordingly, solidification ends within the incubation time of the nucleation, and amorphous fraction is caused in some cases. This amorphous fraction may be included in the term of micro-crystallization, and referred to as micro-crystallization as well in this text.
Techniques of adjusting the output of a laser beam irradiated in the laser annealing process to be smaller than the threshold value of micro-crystallization of an a-Si film are described in Patent Publications JP-A-2000-114174 and JP-A-2002-8976.
FIG. 9 shows analysis of an irradiated area described in JP-A-2000-114174, wherein the irradiated area is analyzed by using an exciting laser 32. This publication teaches that, at first, a preliminary irradiation area is formed by irradiating a one-shot pulse laser beam onto a substrate 31 on which an a-Si film is formed, with the energy density being changed in a pulse-by-pulse basis. Thereafter, the exciting laser 32 is irradiated to obtain reflected scattered light 34 and determine whether or not the micro-crystallization has occurred, by using a Raman spectroscope 33 from the intensity of the scattered light 34 reflected from the poly-Si portions in the a-Si film irradiated with the one-shot pulse laser.
According to the technique described in JP-A-2000-114174, the micro-crystallization threshold value of the a-Si film is checked by this kind of a preliminary irradiation process. An energy density smaller than the micro-crystallization threshold value is then determined as the energy density of a laser beam to be irradiated in the main irradiation process on the a-Si film in the area where TFTs are to be formed in the poly-Si film converted from the a-Si film.
Meanwhile, there has been a proposal for a technique of controlling positions of crystal grains in which a laser beam having an energy density not smaller than the micro-crystallization threshold value of the a-Si film is intentionally irradiated (see JP-A-2003-332346), unlike the general laser irradiation techniques. In this case, determination of the micro-crystallization threshold value is also highly important in order to form crystal grains whose positions are steadily controlled.